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Hybrid Storage Pools (Now with the benefit of hindsight!)

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ahl0003

http://dtrace.org/blogs/ahl/?p=1209 by Adam Leventhal

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Hybrid Storage Pools
         Using Disk and Flash with ZFS
       (Now with the benefit of hindsight!)



Adam Leventhal @ahl
Flash	
  Emerges	
  
•  Storage	
  medium	
  invented	
  in	
  1980	
  
      –    Very	
  fast	
  reads	
  (~50us)	
  
      –    Fast	
  writes	
  (~300us)	
  
      –    High	
  IOPS	
  /	
  low	
  latency	
  
      –    Limited	
  number	
  of	
  write	
  cycles	
  
•  2004:	
  flash	
  cost	
  as	
  much	
  as	
  DRAM	
  
•  2007:	
  flash	
  cost	
  was	
  right	
  between	
  DRAM	
  and	
  disk	
  
Disk	
  is	
  dead…	
  just	
  like	
  tape	
  
•  Many	
  predicted	
  the	
  death	
  of	
  disk	
  or	
  relegaSon	
  of	
  disk	
  to	
  backup	
  
•  Didn’t	
  happen	
  
•  All-­‐flash	
  soluSons	
  sSll	
  trying	
  to	
  gain	
  mass	
  adopSon	
  
ZFS	
  circa	
  2007	
  
•    Sun	
  was	
  developing	
  a	
  ZFS-­‐based	
  storage	
  appliance	
  (Fishworks)	
  
•    ZFS:	
  enterprise	
  class	
  storage	
  on	
  commodity	
  hardware	
  
•    Problem:	
  enterprise	
  storage	
  was	
  a	
  lot	
  faster	
  
•    Looked	
  at	
  tradiSonal	
  soluSons	
  
       –  NV-­‐DRAM	
  to	
  accelerate	
  writes	
  
       –  Massive	
  DRAM	
  to	
  cache	
  reads	
  
•  But	
  it	
  was	
  just	
  the	
  right	
  Sme	
  for	
  flash…	
  
Hybrid	
  Storage	
  Pool	
  (HSP)	
  
•  Use	
  flash	
  as	
  a	
  storage	
  Ser	
  
•  Between	
  DRAM	
  and	
  disk	
  in	
  cost,	
  capacity,	
  latency,	
  throughput	
  
•  Use	
  commodity	
  disks	
  
      –  7200	
  RPM	
  
      –  Good	
  throughput	
  
      –  Great	
  $/GB	
  and	
  wa_s/GB	
  
•  Combine	
  disk,	
  flash,	
  DRAM	
  into	
  a	
  hybrid	
  pool	
  
•  In	
  ZFS:	
  
      –  ZFS	
  intent	
  log	
  (ZIL)	
  for	
  write	
  acceleraSon	
  
      –  L2ARC	
  to	
  extend	
  the	
  reach	
  of	
  the	
  ZFS	
  cache	
  
Hybrid	
  Storage	
  Pool	
  Example	
  

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Hybrid Storage Pools (Now with the benefit of hindsight!)

  • 1. Hybrid Storage Pools Using Disk and Flash with ZFS (Now with the benefit of hindsight!) Adam Leventhal @ahl
  • 2. Flash  Emerges   •  Storage  medium  invented  in  1980   –  Very  fast  reads  (~50us)   –  Fast  writes  (~300us)   –  High  IOPS  /  low  latency   –  Limited  number  of  write  cycles   •  2004:  flash  cost  as  much  as  DRAM   •  2007:  flash  cost  was  right  between  DRAM  and  disk  
  • 3. Disk  is  dead…  just  like  tape   •  Many  predicted  the  death  of  disk  or  relegaSon  of  disk  to  backup   •  Didn’t  happen   •  All-­‐flash  soluSons  sSll  trying  to  gain  mass  adopSon  
  • 4. ZFS  circa  2007   •  Sun  was  developing  a  ZFS-­‐based  storage  appliance  (Fishworks)   •  ZFS:  enterprise  class  storage  on  commodity  hardware   •  Problem:  enterprise  storage  was  a  lot  faster   •  Looked  at  tradiSonal  soluSons   –  NV-­‐DRAM  to  accelerate  writes   –  Massive  DRAM  to  cache  reads   •  But  it  was  just  the  right  Sme  for  flash…  
  • 5. Hybrid  Storage  Pool  (HSP)   •  Use  flash  as  a  storage  Ser   •  Between  DRAM  and  disk  in  cost,  capacity,  latency,  throughput   •  Use  commodity  disks   –  7200  RPM   –  Good  throughput   –  Great  $/GB  and  wa_s/GB   •  Combine  disk,  flash,  DRAM  into  a  hybrid  pool   •  In  ZFS:   –  ZFS  intent  log  (ZIL)  for  write  acceleraSon   –  L2ARC  to  extend  the  reach  of  the  ZFS  cache  
  • 6. Hybrid  Storage  Pool  Example  
  • 7. Hybrid  Storage  Pool  Example  
  • 8. ZFS  Caching   •  AdapSve  Replacement  Cache  (ARC)  as  the  primary  DRAM  cache   •  L2ARC  developed  by  Brendan  Gregg  to  use  external  (flash)  devices   •  Takes  into  account  opSmal  IO  pa_erns  for  flash   –  Random,  small  writes  =  hastened  failure   –  SequenSal,  large  writes  =  happy  SSDs   –  Thro_les  writes  to  preserve  longevity   •  Uses  predicSve  evicSon  to  idenSfy  blocks  to  cache  
  • 9. L2ARC  Problems   •  Non-­‐persistent   –  Aeer  a  reboot  or  fatal  system  failure,  the  cache  is  empty   •  Slow  to  warm  up   –  Will  only  write  to  one  device  at  a  Sme  -­‐>  best  case  1TB  /  hour   –  Real  world  example  2TB  in  24  hours   •  Conceptually  most  of  the  way  there   •  No  real  way  to  tune  it  to  a  workload   •  Not  much  real-­‐world  tesSng  and  tuning  done  
  • 10. Changing  Landscape   •  DRAM  prices  have  dropped  dramaScally   •  Large  memory  systems  available  (3TB+)   •  NAND  flash  is  geing  trickier  to  build  around   •  Endurance  and  performance  decrease  as  lithography  and  price  decrease   –  MLC  and  “TLC”  (volume  flash)  have  parScularly  short  lives   •  Running  into  size  limitaSons   –  32nm  in  2008   –  19nm  today   –  Supposed  floor  around  11nm   •  SSDs  are  becoming  increasingly  complex  
  • 11. What  to  do  today?   •  The  L2ARC  can  help   –  The  SSD  space  is  large  and  highly  varied   –  Generally  cheap,  laptop  SSDs  suffice  for  the  L2ARC   –  Give  it  enough  Sme  to  warm  up  (hours  or  days)   –  Measure  the  impact  on  your  actual  workload   •  The  ARC  is  great  and  relaSvely  simple   –  Load  up  on  DRAM  
  • 12. Next  for  ZFS   •  For  the  L2ARC  to  be  viable,  it  needs  to  be  persistent   •  Lots  of  performance  work  needed   –  Run  it  through  a  bunch  of  real-­‐world  use  cases   –  Make  it  easy  to  collect  coherent,  relevant  data   –  Create  the  right  knobs  for  users  to  turn   •  There  are  a  few  companies  using  the  L2ARC   •  Hopefully  they  will  take  up  the  mantle